MakoShark72

Anybody know the STOCK driven gear tooth count in my 72 sbc/auto/3.08 speedo??

Isn't the DRIVE gear in the tranny, and the DRIVEN gear in the speedo?

I swapped to a 2004R and the websites call for a 13 tooth (white) drive gear, and a 30 tooth (blue) driven gear..

Thanks,
Rob

RIO68

BTO has a chart with the numbers---on their web site that might help you.

MakoShark72

Nope, that chart tells me which gears I need.. My question was what gear comes in a 72 speedo...?

But thanks anyway.

78pace

all most positive Willcox has the charts on his website under tech info

7T1vette

The "drive" and "driven" speedo gears are both in the tranny. The drive gear is a [sleeve-type] worm gear that is placed on the output shaft; the driven gear is placed in the small bore at the top/back of the tranny and it meshes with the drive gear...plus, it receives the speedo cable which is then screwed down to the gear carrier.

As far as determining what speedo driven gear you need...that depends on what drive gear is in the tranny. If there is a gear set in there now, hook it up and drive it on the highway at 60 mph; time each mile and average it to the number of seconds per mile. For every 3 second difference from a time of one minute [ 60 seconds per mile traveled], there needs to be a one tooth difference in the drive gear. A shorter time (less than 60 seconds) requires more teeth; a longer time requires fewer teeth. So, if you are running at 60 mph and it takes you 54 seconds to go exactly one mile, you need to get a gear with 2 more teeth than the drive gear you presently have. Hope that helps figure it out.

MakoShark72

Thanks. That clears it up, as I didnt realize both were in the tranny. I already know that I need a 13 tooth drive and and a 30 tooth driven (from BTO website), just need to verify which ones are currently installed in the tranny.

Are the gears accessible without dropping the tranny pan??

Thanks again!
Rob

mvette76

i believe you have to remove the tail housing to get to the drive gear

MakoShark72

I seem to recall that you do (remove the tail housing) for the 700R4, but I was thinking the 200 was different...

More research...

7T1vette

You shouldn't need to change the drive gear. Just buy the correct match for whatever drive gear you have to get the right ratio for the speedo. There are more than one combination of gears available for a 3.08 gear set.

JimT

If you have a TH400, you can remove the speedo cable and the driven gear housing and colored gear. Look inside the tranny at the drive gear, mark a spline, have someone turn a rear wheel one full turn and count the number of splines. A good tranny repair shop when given the drive gear size (number of splines), rear end ratio, and tire size can tell you what color gear to use.

7T1vette

It's just a lot easier to 'adjust' the driven gear for the speedometer error than going to that much trouble. [IMO]

JamesPDX

Lower the drive line end from the trans, take the rear housing off the trans, R&R the driven gear. It's that simple. You may need to R&R the sleeve if the gear is too large, but in your case, I do not think so.

The Best to you,

I need a 33 tooth driven gear to compensate for my 3.42 rear axle.
Can anyone help me to locate this gear?
I've tried many places and sites.
I ordered one in stock from GM Parts Direct that they had, but was cancelled when it became unavailable (in 5 days).
If I find it, I'll share the location.

My approximate original setup
Rear axle gear ratio 2.72
Tire diameter in inches 26.40
Ratio Required 0.4817 (13 x 27 gear set => 13/27 = .4814)

My new setup
Rear axle gear ratio 3.42
Tire diameter in inches 27.40
Ratio Required 0.3976

The following are my Preferred Gear Combinations
Drive Driven Ratio
10 25 0.4000
11 28 0.3929
12 20 0.4000
13 33 0.3939 (my original was a 13 x 27 set)
14 34 0.4000
15 38 0.3947
16 41 0.3902
17 43 0.3953
18 45 0.4000

I can therefore, utilize most any one of these gear combination.
The least costly would be the 33 driven gear, if I can find it.
These gears are getting really hard to find.

Thanks,
JamesPDX
Portland, Oregon

Mooser

I went to the local transmission shop (private shop, not a chain, been around forever)

I needed a 43 driven, the guy goes into the back storage room and a few minutes later comes out with a virtually new gear, I think he charged me $5

Check your local trans shops, walking in might be better than phone-calls
HIH
Mooser

JamesPDX

Thanks,

I've contacted a few local, in other states, some GM on-line parts places, and a couple of specialty shops on-line. There are so many that I thought it might be good to ask for help. I'll keep checking the local shops: there are more. Most everywhere, the item is either discontinued or out of stock.